Introduction

The Medusa Radiometrics rhoC5 measures field bulk density and soil moisture content directly in the field. Field bulk density is an important physical soil health indicator and is generally used for assessing soil quality. The rhoC5 measures the backscatter of gamma radiation emitted from a (low-activity) source. The sensor can measure up to 1 meter deep and only needs a hole of 30 mm diameter for such a measurement. With this sensor, there is no need for a soil pit or any laboratory work to determine soil bulk density. The sensor provides the measured densities during measurement in the field. This allows the soil scientist to gather more information on the vertical as well as the horizontal scale.

The housing is rugged, easily portable, and can be operated by any field worker. 

The rhoC5 density meter is controlled over WiFi by an app on an android smartphone. The data acquisition is monitored by the smartphone and is stored locally on the phone. The user of the app can export the data from the smartphone to a PC, or save it in the cloud.

The rhoC5 contains three sensors: a density sensor, a depth sensor, and a soil moisture sensor. The density is determined over a layer of approximately 50 mm. This range is located between 38 and 88 mm above the bottom of the titanium section. The moisture sensor is located in the nylon tip of the rhoC5. The moisture sensor measures the soil moisture content over approximately 50 mm. This range is located between 32 and 82 mm below the top of the nylon section. As a result, the center of the moisture measurement has an offset of 130 mm with respect to the density measurement. The depth sensor uses radar technology and returns the depth of the center of the density measurement.

Working principle of the density sensor

The figure above demonstrates schematically how the density measurement works. The small red disk contains a radioactive ²²Na source with a low activity below the exemption value. This source will radiate gamma-rays into the soil. This soil will interact with these gamma-rays. One of the interactions that can occur is known as Compton scattering, where the direction and energy of the gamma-ray is altered. The dark gray cylinder in the figure is a scintillation crystal that can detect gamma-rays when they interact with the crystal. Between the crystal and the source, a piece of tungsten is positioned. Tungsten has a very high density, which prevents most radiation from the source from reaching the crystal directly. As a result, most radiation registered by the scintillation crystal has been scattered in the soil surrounding the rhoC5. With increasing soil density, more radiation will be scattered, and more gamma-rays will be registered. The rhoC5 has been calibrated to determine the soil density based on the number of gamma-rays detected per second.

Measurement procedure

To perform a complete measurement on a new location, you first have to drill a hole into the soil with a diameter of 30 mm up to the maximum depth you are interested in, plus at least 20 centimeters to compensate for the part of the tool which extends beyond the density measurement position. A hand auger or other coring device should be used to drill the hole. Both the density and moisture measurements consider a limited volume of soil, not extending beyond a few centimeters from the rhoC5. Therefore, it is vital to have a close-fitting hole. A hole that is too wide will result in underestimated values of both soil density and soil moisture content.

Use the provided plastic base to keep the rhoC5 from sliding down into the hole. Use the plastic base even when the soil will hold the rhoC5 in place, as this will provide a good reference for the depth sensor. The depth sensor is calibrated to use the bottom of the plastic base as the top of the soil.

In the footer of the app that accompanies the rhoC5, the current depth of the density meter is visible. This makes it easy to position the sensor at the desired depth. During a measurement, the app automatically logs the depth and soil density. Depending on the required accuracy, the app will estimate the measurement's progress and notify you when the measurement is completed.

The rhoC5 continuously collects data and runs a stabilization algorithm to compensate for temperature fluctuations. A density measurement can be started only when the initial stabilization has been completed, and this may take up to five minutes. To ensure the rhoC5 is ready for immediate use and to bypass this initial waiting period, it is recommended to keep the device powered on during transitions to new locations or while preparing the measurement site.

Measurement lay-out

All data is collected in a single project. The project contains a series of measurements, where each measurement describes a single borehole. The measurement stores the position (GPS coordinates retrieved from the rhoC5) and a series of readings. Each reading contains a set of data, which includes depth, soil density, and soil moisture content.

Note that the depth corresponds to the depth of the density meter. The moisture sensor is located 130 mm lower.

The radioactive source

The rhoC5 contains a radioactive ²²Na source, which is constantly emitting gamma radiation and cannot be switched off. The activity of this source is below the exemption value, and no permit is required to operate and store the device. Nevertheless, it is important always to keep this in mind and reduce exposure to a minimum. The ALARA principle can be used for this, which stands for as low as reasonably achievable. Reduce the proximity of the source as much as possible. This means storing the device at a distance from people and using shielding, such as a wall, when possible. Try to reduce the time in proximity to the rhoC5 as much as possible. When preparing the next measurement, leave the rhoC5 pushed down into the previous borehole. The borehole will shield the radiation and prevent exposure.

Due to decay, the activity of the source will reduce over time. This is a physical property independent of whether the rhoC5 is being used. A lower source activity will result in a measurement taking longer to complete. It is advised to have the source replaced every 5 years.

More information on the radioactive source can be found on the Safety note page.